Hello Alan, Tim, Jim, and gang,
Just thought I would provide a little update, and also a very interesting
observation regarding the problem at Mikes (W9RE) in which his stubs or
band pass filters for that matter have no impact when transmitting on 40
meters while listening on 20 meters (same issue on other band combinations
too). I will keep this somewhat brief for now as there is still a lot more
work to be done to uncover all his underlying problems but we at least have
a specific task that must be accomplished before we dig deeper.
A month ago I had never dealt with stubs or PIM (passive intermodulation
distortion) and only had limited experience working with active IMD in
regards to overdriving mixers, nevertheless I now know much more about
these subjects than I did a month ago.
The biggest thing we discovered is that the Raspy sound everyone talks
about when listening to their transmitter up on the harmonic band most of
the time is probably the result of mixing with 60 hertz, and the only way
we were able to figure this out was to use my cheap laptop based SDR
receiver zoomed in on the RF spectrum display while using the minimum RBW
(resolution bandwidth) available which in my case was 7.6 Hz.
We have done too many experiments to discuss online (yes we shut off all
power to the house including back up battery systems, yes we did transmit
using battery power, yes we did place stubs at high impedance points
relative to the antenna and transmitter, yes we tried bandpass filters on
the transmitter, etc.), but so far what we have found is that if you have a
receive antenna close to your house the chances of your signal sounding
Raspy on the harmonic band is tremendous and if you look at the signal on
each side of the center frequency signal on the harmonic band you will see
what appears to be sidebands extending outward at intervals of 60, 120, ...
Hz (I hate to call them sidebands in this case but that term provides a
good picture of what you will see). Also the signal strength of these
harmonic sidebands do not necessarily track the signal strength of the
center frequency on the harmonic band when rotating portable direction
finding antennas (sometimes the sidebands go up while the center frequency
signal goes down, etc). This phenomena undoubtedly deals with passive as
well as active mixing with all the components/contents of a home and it
appears almost impossible to eliminate and I was even able to duplicate
this problem running 2 watts in my mobile parked out in the street in front
of my house with my portable SDR receiver located near my house, and
duplicated this same issue at Mikes (it sure is not unique to just Mikes).
Now the next question will be if we eliminate sources of mixing out around
Mikes antenna farm can we reduce the mixing with 60 Hz that he detects when
listening on the harmonic band using his tower mounted antennas, and we
hope the answer is yes, and then stubs and bandpass filters will have a
chance. We have to eliminate the mixing products, otherwise stubs and
bandpass filters on his 40 meter transmitter will be useless for solving
his local receive problem on the harmonic band (as others had already
mentioned/speculated). I'm sure we still have a lot more to learn and
uncover but seeing how pervasive the mixing with 60 Hz is has been eye
opening.
The immediate action item for Mike (W9RE) is to repair or remove his 40
meter Moxon on one of his 5 towers as it's SWR jumps around a considerable
amount when transmitting, and we also find 3rd order IMD products the
strongest and also erratically changing amplitude a large amount when near
this tower even when the 40 meter Moxon is not in use (not being used as a
transmit antenna). We are hoping that some or all of the mixing issues are
related to his ill Moxon.
Just FYI,
Don (wd8dsb)
On Sun, Dec 5, 2021 at 7:50 PM Mike Wetzel <mjwetzel@comcast.net> wrote:
> I have been fighting a bad 2nd harmonic problem for a number of years
> mainly
> from 40 to 20 meters. A few years ago a double stub on 40 only reduced the
> 2nd harmonic about 15db so I thought something else was going on. This
> year
> the double stub has no effect at all. Others have suggested it is a
> reradiation thing. The 2nd harmonic note is not pure but the raspy, growly
> thing that Frank (W3LPL mentions). Don, WD8DSB has been helping me
> remotely
> on a weekly basis to try and figure out what is going on. This week he
> came
> over and used his SDR, portable radio, loops, portable flag (March 2021
> cover QST) and initial data suggests strong 2nd harmonic content coming
> from
> the direction of my 20 meter tower.
>
>
>
> Setup: we temporarily used a low 40 meter dipole as a transmit antenna
> with
> feedline going directly to my transmitter and with Don in the field certain
> orientations of his flag antenna showed some reduction in 2nd harmonic when
> the stubs are inline but other orientations showed no reduction with the
> stubs inline which appears to indicate the stubs really are working but
> something else is generating strong harmonics on 20 meters when
> transmitting
> on 40 meters. My 20 meter tower is 170' of Rohn 45 with a 3 stack of 48'
> boom 20 M OWA antennas. Antennas utilize grounded elements (except DE) and
> use a 6 turn coax choke balun. Top one is at 170' on a mast with a prop
> pitch in line rotator, middle one is at 110' on a swing arm with prop pitch
> and the lower one is at 50' again on a swing arm with a prop pitch rotator.
> At 90' on this tower is a W6NL/W2SC 40 M Moxon fixed SE. The Moxon right
> now has an intermittent connection somewhere as the SWR will fluctuate
> slightly during transmit. Right now there is no bonding between the mast
> or
> the side arms to the tower. It is on my list to do ASAP.
>
>
>
> N7WY has suggested to use higher harmonics and smaller yagis to try and
> isolate a particular area. Later this week we plan on doing thorough
> measurements of the 2nd harmonic out in the field looking for location of
> maximum signal strength, etc. We also plan on using VHF direction finding
> gear to see if we can hear the signal up on the higher harmonic frequencies
> as others have suggested.
>
> I am wondering if anyone else has experienced a problem like this and has
> any ideas?
>
>
>
> Also posting on Towertalk reflector.
>
>
>
> Thanks,
>
>
>
> Mike W9RE and Don WD8DSB
>
>
>
>
>
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